The present invention relates to technology for managing device controllers, and more particularly to managing resource reclamation (or “scavenging”) in data storage systems.
Many file systems and storage subsystems provide a facility for recovering deleted files. A simple example of this would be the Microsoft® Windows® operating system's Recycle Bin where, if the user accidentally deletes a file, they can go to the Recycle Bin, list the deleted files and then select the file(s) that they want to recover. (Microsoft and Windows are trademarks or registered trademarks of Microsoft Corporation in the United States and elsewhere). The file is then made available again to the user. In this environment, the Recycle Bin will gradually fill up and free space on the system will eventually run out. Windows today monitors the free space and under some circumstances offers the administrator a prompt along the lines of “you are running out of space—do you want to empty the recycle bin to make some more”—which the administrator must explicitly accept in order to free up space.
When this concept is extended to a virtualised storage environment, volumes are saved in a Recycle Pool and, at some point, free extents run out. When this occurs, the creation of new volumes will fail and users must manually select which volumes to permanently delete from the Recycle Pool to free up extents. After doing this, they can retry the volume create command and, if enough extents are now free, the command will succeed.
Late allocated volumes can be created in a virtualised storage environment. When they are created, they have no physical extents allocated. Host writes to unallocated extents cause the system to dynamically allocate physical extents. Host reads to unallocated extents must return all zeros. To ensure that host writes can always be honoured, the system needs to keeps a pool of zeroed unallocated extents (extents that have been cleared of data) available. If the pool were to become empty the system would not be able to allocate an extent for a write and would have to fail back to the host and take the late allocation volume offline. In this situation there is no opportunity for a user to step in and manually delete volumes from the recycle bin to free up extents. This kind of manual intervention is also unacceptable in a large system with many administrators and no single point of control.